Fix potential crash when entire span request is received.

Fix potential crash when entire span request is received.

The variable index used in this scenario for accessing the dahdi_pvts was
wrong and was most likely copied from the several other places it is used
correctly.

(closes issue #15998)
Reported by: tsearle
Patches:
     dahdi_reset_crash.patch uploaded by tsearle (license 373)

Modified:
   branches/1.4/channels/chan_dahdi.c

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.6.0@222395 65c4cc65-6c06-0410-ace0-fbb531ad65f3